This invention relates to dynamoelectric machines and in particular to an electronically commutated motor, a rotatable assembly for an electronically commutated motor, and methods of operating the electronically commutated motor and rotating the rotatable assembly in which the permanent magnet elements thereof provide varying magnetic flux to a stationary assembly as the rotatable assembly rotates.
Various circuit and motor designs have been utilized in the past to develop various types of brushless DC motors, as exemplified in U.S. Pat. No. 4,005,347 issued Jan. 25, 1977, U.S. Pat. No. 4,015,182 issued Mar. 29, 1975, and U.S. Pat. No. 4,449,079 issued May 15, 1984, each of which are incorporated herein by reference. In general, such brushless DC motors have a stator with a plurality of windings therein, a rotor with a plurality of constant magnetic polar regions having constant flux density, and sensors for sensing the relative position of the rotor polar regions with respect to the stator. Signals developed by the position sensors were processed by circuitry for selectively energizing the windings of the motor.
In some of the electronically commutated motors, the circuitry for selectively energizing the windings of the motor includes a pair of driving transistors, such as field effect transistors (FETs), for each winding or a total of six FETs in the circuitry. This circuitry is referred to as a full bridge inverter drive circuit. Since FETs are expensive, other electronically commutated motors have been developed, such as the motor shown in U.S. Pat. No. 4,449,079 mentioned above. In general, such other motors have the circuitry for selectively energizing the windings of the motor which includes only a single driving transistor for each winding or a total of three FETs in the circuitry. This circuitry is referred to as a half bridge inverter drive circuit.
In a three-phase distributed winding motor having a full bridge inverter drive circuit, current can be supplied to each winding by the pair of driving transistors for typically 240 electrical degrees per cycle. However, in a three-phase salient pole motor having a half bridge inverter drive circuit, current can be supplied to each winding by the driving transistors for at most 120 electrical degrees per cycle. This is because the back electromotive force (EMF) waveform of each winding of a three-phase salient pole motor having a half bridge inverter drive circuit has a flat top width of at most 120 electrical degrees (i.e., while the back EMF is positive for 180 electrical degrees) which is the period that current is applied to the winding. Additionally, in a motor having a stator wound with a distributed winding and a half bridge inverter drive circuit, current can be supplied to each winding by the driving transistors for at most 180 electrical degrees per cycle. This is because the back EMF waveform of each winding of a motor having a stator wound with a distributed winding and a half bridge inverter drive circuit has a flat top width of at most 180.degree.. However, it would be desirable in either of the above motors having a half bridge inverter drive circuit to have a back EMF waveform with an increased flat top width, such as 240 electrical degrees, so that torque pulsations can be minimized. This would permit a motor having a half bridge inverter drive circuit, which costs less than a full bridge inverter drive circuit, to operate similarly to a motor having a full bridge inverter drive circuit.